设f(n)=nn+1,g(n)=(n+1)n,n∈N*.(1)当n=1,2,3,4时,比较f(n)与g(n)的大小.(2)根据

2025-06-20 23:13:59
推荐回答(1个)
回答1:

(1)当n=1时,nn+1=1,(n+1)n=2,此时,nn+1<(n+1)n
当n=2时,nn+1=8,(n+1)n=9,此时,nn+1<(n+1)n
当n=3时,nn+1=81,(n+1)n=64,此时,nn+1>(n+1)n
当n=4时,nn+1=1024,(n+1)n=625,此时,nn+1>(n+1)n
(2)根据上述结论,我们猜想:当n≥3时,nn+1>(n+1)n(n∈N*)恒成立.
①当n=3时,nn+1=34=81>(n+1)n=43=64
即nn+1>(n+1)n成立.
②假设当n=k时,kk+1>(k+1)k成立,即:

kk+1
(k+1)k
>1
则当n=k+1时,
(k+1)k+2
(k+2)k+1
=(k+1)?(
k+1
k+2
)k+1
(k+1)?(
k
k+1
)k+1
=
kk+1
(k+1)k
>1
即(k+1)k+2>(k+2)k+1成立,即当n=k+1时也成立,
∴当n≥3时,nn+1>(n+1)n(n∈N*)恒成立.